Add a spin loop hint for Arc::downgrade

Author: nicbn

Diff for: library/alloc/src/lib.rs

@@ -118,6 +118,7 @@
 #![feature(raw_ref_op)]
 #![feature(rustc_attrs)]
 #![feature(receiver_trait)]
+#![feature(renamed_spin_loop)]
 #![feature(min_specialization)]
 #![feature(slice_ptr_get)]
 #![feature(slice_ptr_len)]

Diff for: library/alloc/src/sync.rs

@@ -10,6 +10,7 @@ use core::cmp::Ordering;
 use core::convert::{From, TryFrom};
 use core::fmt;
 use core::hash::{Hash, Hasher};
+use core::hint;
 use core::intrinsics::abort;
 use core::iter;
 use core::marker::{PhantomData, Unpin, Unsize};
@@ -764,6 +765,7 @@ impl<T: ?Sized> Arc<T> {
         loop {
             // check if the weak counter is currently "locked"; if so, spin.
             if cur == usize::MAX {
+                hint::spin_loop();
                 cur = this.inner().weak.load(Relaxed);
                 continue;
             }